The present invention relates to an image recording apparatus capable of recording an image of half-tone, and more particularly to an image recording apparatus capable of controlling a gray level of an image for each dot, i.e., each picture element, by using a thermal head, for example.
An image recording apparatus for recording image data by using a thermal head in which heat is selectively generated in the thermal head and characters are printed without any impact on a recording paper by making use of the selectively generated heat. The image recording apparatus of the nonimpact type can print an image quietly, while noisy printing is unavoidable in the image recording apparatus of the impact type in which characters are printed on a recording paper by mechanical impact. Further, the image recording apparatus using a thermal head is superior to the other nonimpact type image recording apparatuses, such as an ink jet printer and laser printer, in that the former may be manufactured with smaller size and lower cost.
In the image recording apparatus using a thermal head, heat energy of each heating element contained in the thermal head may be varied. Therefore, if heat sensitive recording paper is used, such as a recording paper of the heat color-developing type, an electrically heating sheet, and the like, a gray level of a recorded image can be controlled for each dot.
FIG. 16 shows a structural representation of an image recording apparatus using an electrically heating sheet as the recording medium. The electrically heating sheet 1 is made up of a resistor layer 2, a support layer 3, and a heat-melting ink layer 4. The support layer 3 supports the ink layer 4 for the resistor layer 2. The support layer 3 may double as the resistor layer 2, if necessary.
In operation, a record electrode 5 and a common electrode 6 are made to contact with the resistor layer 2 of the electrically heating sheet 1. A voltage of a pulse width corresponding to a record pattern is generated by a signal voltage generator 7, and is applied to the record electrode 5. The applied voltage causes a current to flow from the record electrode 5 through the resistor layer 2 to the common electrode 6. Generation of Joul heat by the current occurs right under the needle-like record electrode 5. The generated Joul heat transfers through the support layer 3, reaches the heat melting ink layer 4 to melt the ink contained therein, and the molten ink is transferred onto a recording paper 8.
If a heat value of a heating pulse is varied by changing a duration of the pulsative voltage applied to the electrode 5, a molten area of the ink of the layer 4 varies proportionally to the heat value of the applied heat energy. In other words, a diameter of each print dot may be changed by varying a time width of a pulse applied to each record electrode. In the sublimation type heat transfer, which uses an ink doner film with a sublimation ink coated surface, an amount of transferred ink is controllable in accordance with an applied heat energy. Thus, the image recording apparatus described above may record an image on a recording paper with discrete gradations in gray level controlled for each dot.
The applicant in the present patent application proposed an image recording apparatus based on the above principle. The apparatus is capable of recording a desired halftone image by controlling an accumulative time of the pulse applied to each heating element of a thermal head in accordance with a desired gradation of tone.
FIG. 17 shows a schematic illustration of the proposed image recording apparatus. As shown, gradation data 21 representing a gradation of one picture element (pixel) is inputted to a gradation to bit-train converting table 22.
This converting table 22 is realized by a read only memory (ROM), for example. A bit train consisting of consecutive "1's", which expresses the gradation data 21, is prestored in the storage location of an address assigned to the data 21 in the ROM. More specifically, when it is assumed that a tone can be expressed in terms of eight gradations, the data of the 8th gradation, which is highest in the tone, consists of eight consecutive "1's" bits "11111111". Such data is stored in the storage location of an address assigned to the data. Likewise the 7th gradation data is expressed by seven consecutive "1's" and one "0", "11111110".
The data signal of 8-bit long are transferred in parallel and inputted to a bit-train to pulse-train converting means 23. The converting means 23 converts the 8-bit data into a pulse signal corresponding to the number of "1's", and applies the converted pulse signal to a corresponding heating element 24, which is provided corresponding to the pixel whose gradation data is under process. Therefore, a value of heat generated by the heating element 24 is controlled in accordance with the gradation data 21, so that the pixel thus recorded on the recording paper has halftone.
The above recording apparatus is advantageous in that a tone can be set at any desired gradation by merely changing the contents in the gradation to bit-train converting table, and the resultant halftone image is high in quality. It has a disadvantage given below, however. The number of heating pulses to the heating element 24 is selected to be proportional to the number of gradations indicated by the gradation data 21. Therefore, when an image in high gray levels, or of black is recorded, the heat color developing sheet used would be thermally deformed. A deformation of the sheet provides poor contact or noncontact of the sheet with a recording paper, which is moving during the recording operation. This creates serious problems, such as partial blur or omission in the resultant image.